The goal of this proposal is to understand the synaptic mechanism and neuronal circuitry underlying complex visual processing in the mature mammalian retina, with a focus on the function and organization of individual synapses in the direction selective circuit. The proposed study is based on recent findings that identified critica synaptic interactions underlying the generation of direction selectivity. These findings suggested a previously unappreciated level of synaptic intricacy in the underlying neuronal circuit. They revealed the importance, as well as the possibility, to understand the mechanism of directional computation at a microcircuit level. In order to gain direct knowledge of the function and organization of the direction-selective microcircuits, a novel experimental approach is proposed here, which integrates two-photon imaging, dual patch-clamp recording, spot UV uncaging, and transgenic technology, so that neuronal connectivity and interactions at individual synaptic sites can be measured in an intact retinal network and correlated with the morphological and functional properties of the cells in the same experiment. The proposed experiments are designed to understand (1) the synaptic mechanism of cholinergic transmission in the mature retina, (2) the functional organization of individual cholinergic and GABAergic synapses between starburst amacrine and direction-selective ganglion cells, (3) the functional organization of GABAergic synapses between starburst amacrine cells, and (4) the synaptic interactions at bipolar cell axon terminals. Results from these experiments are expected to provide novel insights into the nature of dendritic and axonal computation at a synaptic level and in an intact retinal circuit. This approach may also provide a novel experimental paradigm for studying the function and connectivity at individual synapses in other CNS circuits.